The present invention relates an apparatus for measuring the moisture content of a target, and more particularly, to a moisture measuring device with a focussing probe.
The use of electromagnetic field of microwave frequency to measure the moisture content of various targets is well known. See, for example, U.S. Pat. Nos. 3,441,844, 3,811,087, 3,815,019, and 4,052,666. See also, Great Britian Pat. No. 1,354,474. In an article entitled "A Non Destructive Method For Measuring The Complex Permittivity Of Dielectric Materials At Microwave Frequencies Using An Open Transmission Line Resonator" which appeared in IEEE Transactions On Instrumentations And Measurement, volume 1m-25, No. 3, September 1976, the authors, Eiji Tanabe and William T. Joines described a moisture measuring apparatus using electromagnetic field of microwave frequency and a resonating cavity to guide the microwave onto the target.
U.S. Pat. No. 4,013,065 teaches a moisture measuring apparatus to measure the moisture content of the stratum coreum of human skin by means of a stray field capacitor in an RC network with a 9 KHz oscillator as a source of electromagnetic field. The teaching of that patent, however, is the use of the principle of a "stray field capacitor" to measure the moisture content of the human skin. In the theory of operation, a resistor and a capacitor form a voltage divider for the output of the 9 KHz oscillator, with the junction of the capacitor and resister forming the pickup point for signal processing. The capacitor is placed against the human skin. The stray field of the capacitor penetrates the human skin and is influenced by the moisture content of the skin. The amount of moisture causes an increase in the probe capacitance. The resultant increase in capacitance causes an increase in current in the circuit formed by the series of the resistor and probe capacitor. This increased current causes a greater voltage drop across the resistor and a lower voltage at the junction of the resister and the capacitor. This change in voltage is the transduced signal output used for indicating the amount of moisture. However, the use of a measuring device operating at 9 KHz can result in ambiguous readings. At 9 KHz, many other materials such as: salts, proteins, charged cell membranes, and other solutes may also affect the stray capacitance of the probe capacitor, and thereby after the measurement of the moisture content.
A paper entitled "Linear Measurement of the Water Content of The Stratum Corneum of Human Skin Using a Microwave Probe", by the applicant was published on Oct. 6, 1979 in the IEEE Engineering in Medicine and Biology Society 1st Annual Conference held in Denver, Colo., relating to the present invention.